​Musical U: This website is headed by a great music educator named Christopher Sutton. It's an outstanding resource for all things related to Music Theory, including ear training, scales, chord building, the Circle of Fifths, and more! It's subscription based, but there's tons of free resources!

​​​​Scott's Bass Lessons: Scott Devine is a masterful bass player who has an excellent subscription-based website. He also has some free tools on his website, and lots of free videos on his YouTube Channel, where he has over 400K subscribers!